This invention relates to a method to compensate for the magnetic field heterogeneity inside of an object of investigation inside of a magnetic resonance device, comprising:
(A) determining experimentally or theoretically spatial field distributions and temporal switching characteristics of field manipulating coils present in the magnetic resonance (MR) apparatus including B0 offset coil, linear gradients, non-linear encoding fields and shim coils of first and higher orders;(B) obtaining an uncorrected magnetic field distribution of the object of investigation by measuring (i) the heterogeneity of the static magnetic field and (ii) the heterogeneity caused by susceptibility variations associated with the object of investigation;(C) executing an MR sequence on the MR device with a desired k-space coverage by applying one or several RF pulses to generate a transverse magnetization within the object of investigation, switching currents in the field manipulating coils to effect a primary spatial encoding and recording MR signal data;(D) dynamically updating one or several magnetic field shimming parameters such as carrier frequency shift, offsets of the primary encoding fields or currents applied to the field manipulating coils;(E) performing reconstruction of the MR signal data acquired in step (c) to produce one or more images or localized spectroscopic data.
A state-of-the-art method with these features has been described in {24}, where the magnetic field heterogeneity inside of an object of investigation inside of a magnetic resonance device is compensated over an extended volume by dynamically updating the magnetic field. This method is also called dynamic shim updating. In contrast to static shimming, where one optimal shim set is determined for the whole volume, the method described in {24} allows one to apply optimal shimming parameters consecutively to each excited slice, thereby improving image quality. In this description, this method is called inter-slice shimming. However, this method does not allow one to improve the image quality by updating the shimming parameters during the acquisition of a single slice. This latter method is called here intra-slice shimming.
The reason for this limitation is the fact that in state-of-the-art methods like {24}, the signals that originate from different sub-volumes in the object are synchronized, i.e., the signal echoes contain signals from all excited sub-volumes simultaneously. Therefore, shim updates usually affect all signal contributions at once. To better understand this problem, consider shim parameter optimization for a sub-volume of a 2D spatially encoded slice or 3D spatially encoded slab. In these situations, the local magnetic field homogeneity in the sub-volume would be improved only to its deterioration elsewhere. That is to say that an improvement in image quality would be expected in the sub-volume and a deterioration of the image quality would result outside of this sub-volume. This problem is not solved by dynamically updating the shimming parameters during the acquisition for the second sub-volume in the slice because of two reasons. First, the image quality might be improved in this second sub-volume, however, it deteriorates for the first sub-volume in the slice. Second, data inconsistencies evolve with adverse effects on the whole image quality.